SN Certified Technician

Well ill go with what your saying about the slight sticking at 1,500 rpms. its kind of annoying but it'll work not the worst thing the car could be doing.

I'm just confused cause sometimes after it sticks at 1,500 for a few seconds it goes down to 1,000rpm and sometimes it will go to 700-800 which is idle. it doesnt always go down to 700-800 (idle).

Also, when i start the car after it sat for hours, engine is cold, the car wants to die unless i keep giving it gas, but i think that means the IAC is bad, but the IAC is brand new just got it yesterday.

Thanks again for all you guys help so far.

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There is still a difference. As you saw, mine will slow down at 1500 RPM for a tick. It's not a situation where it hangs and it's for a fraction of a second. The falling of RPM simply slows down as it approaches 1500 RPM, and then it falls back to idle. This was where I was thinking others could offer advice about how their particular cars act, as not every car will fall to idle as mine does (or yours does).

The car wanting to stall when cold is still problematic. Perhaps JRcihker has further info or you can revisit his old posts about checking the IAC wiring to ensure it's doing what it should. I'd be going off the cue that the IAC doesnt work when cold. Once that's sorted out, I'd worry about the hang (because fixing the primary issue will likely fix the secondary [hang]).

SN Certified TechnicianFounding Member

ok i know i check the voltage and everything going to the IAC. IAC is clean as hell because its BRAND NEW. I went through all his steps on the checklist and still having the problem.

JRITCHER please reply if you can to help me figure this out..

thanks alot buddy.

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I haven't been following your efforts very closely, so I may have missed something.
A short recap of what you have tested, adjusted and replaced and the order that you
did the work would be very helpful. That way I won't advise you to duplicate something
you have already done.

SN Certified TechnicianFounding Member

My car idle has been sticking at 2,000 rpm again after it hasnt done it for weeks.
After I cleaned everything. it only did it a few times but still does it until i turn car off and on.
As soon as i turn car back on it's not sticking anymore.

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That's a TPS kind of problem. I don't know what to tell you but to check the TPS voltages when it is acting up.
Check the TPS ground carefully.

Some basic checks you can make to be sure that the sensor is getting power & ground: Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on
will give false readings and possibly damage the meter.
Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on
the EGR and the same wire on the TPS. It should be less than 1 ohm. Next check the resistance between the
black/white wire and the negative battery cable. It should be less than 1.5 ohm.

That's a TPS kind of problem. I don't know what to tell you but to check the TPS voltages when it is acting up.
Check the TPS ground carefully.

Some basic checks you can make to be sure that the sensor is getting power & ground: Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on
will give false readings and possibly damage the meter.
Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on
the EGR and the same wire on the TPS. It should be less than 1 ohm. Next check the resistance between the
black/white wire and the negative battery cable. It should be less than 1.5 ohm.

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I will do that next time it acts up.

What about the codes im still getting even though i've replaced the sensors and such?

with the car sticking at 1500 while comming to stop is built into the programming on the computer. it was made to not drop the rpm immediately and cause more strain on the trans and engine. aem actually has this as a programable option where it wont do it after you go below rolling at 25mph. i believe this is based exactly off of original program.

SN Certified TechnicianFounding Member

Sorry I didn't read your post more carefully. The extra codes are a definite clue to your problem.

CODE: 31 (KOEO) - EVP circuit below minimum voltage. Vref (5 volt reference voltage supplied
by the computer) missing or broken wire or bad connection in circuit. Use a DVM to check for
5 volts on the orange/white wire. If it is missing, look for +5 volts at the orange/white wire
on the TPS or MAP sensor located on the firewall near the center of the car. Use the black/white
wire for the ground for the DVM.
With the sensor removed from the EGR and still connected, press the plunger and watch
the voltage change on the brown/lt green wire. Pull the passenger side kick panel and
measure the voltage at the computer. You will need to remove the plastic cover over
the wires and probe them from the backside. A safety pin may prove very useful for this task.
Use pin 27, EVR input (brown/lt green wire) and pin 46, signal ground (black/white wire) to
measure the voltage. The orange/white wire is Vref and should always be 5 volts -/+ .25 volt.
Be sure to measure Vref at the EGR sensor to rule out any broken wires or bad connections.

Code 51 Engine Coolant Temperature (ECT) sensor signal is/was too high -
Possible bad ECT sensor, or wiring. Possible missing signal ground –
black/wire wire broken or bad connection. With the power off, measure the
resistance between the black/white wire and battery ground. You should see
less than 1 ohm. Check the same black /white wire on the TPS and MAP
sensor. More than 1 ohm there and the wire is probably broken in the harness
between the engine and the computer. The 10 pin connectors pass the
black/white wire back to the computer, and can cause problems.

Pin 7 on the computer - ECT signal in. at 176 degrees F it should be .80 volts

Voltages may be measured across the ECT by probing the connector from the rear.
Use care in doing it so that you don't damage the wiring or connector.

Code 54 – ACT sensor out of range. Broken or damaged wiring, bad ACT sensor.
Note that that if the outside air temp is below 50 degrees F that the test for the ACT can be in error.

Check the resistance of the black/white wire to battery ground. If it is less than 1.5 ohm,
it is good. If it is more than 1.5 ohm, the black/white wire has bad connections or a broken wire.
Always take resistance measurements with the circuit powered off.

Then check the resistance of the ACT sender located in the #5 intake runner on most 5.0 stangs.

ACT & ECT test data:

The ACT & ECT have the same thermistor, so the table values are the same

Pin 7 on the computer - ECT signal in. at 176 degrees F it should be .80 volts

Pin 25 on the computer - ACT signal in. at 50 degrees F it should be 3.5 volts. It is a good
number if the ACT is mounted in the inlet airbox. If it is mounted in the lower intake manifold,
the voltage readings will be lower because of the heat transfer. Here's the table :

Code 66 MAF below minimum test voltage. Insufficient or no voltage from MAF.
Dirty MAF element, bad MAF, bad MAF wiring, missing power to MAF. Check for missing
+12 volts on this circuit. Check the two links for a wiring diagram to help you find the red wire for
computer power relay switched +12 volts. Check for 12 volts between the red and black
wires on the MAF heater (usually pins A & B). while the connector is plugged into the MAF.
This may require the use of a couple of safety pins to probe the MAF connector from the back side of it.

There are three parts in a MAF: the heater, the sensor element and the amplifier. The heater
heats the MAF sensor element causing the resistance to increase. The amplifier buffers the
MAF output signal and has a resistor that is laser trimmed to provide an output range
compatible with the computer's load tables.

The MAF element is secured by 2 screws & has 1 wiring connector. To clean the element,
remove it from the MAF housing and spray it down with electronic parts cleaner or non-inflammable
brake parts cleaner (same stuff in a bigger can and cheaper too).

The MAF output varies with RPM which causes the airflow to increase or decease. The increase
of air across the MAF sensor element causes it to cool, allowing more voltage to pass and
telling the computer to increase the fuel flow. A decrease in airflow causes the MAF sensor
element to get warmer, decreasing the voltage and reducing the fuel flow. Measure the MAF
output at pins C & D on the MAF connector (dark blue/orange and tan/light blue) or at pins
50 & 9 on the computer. Be sure to measure the sensor output by measuring across the
pins and not between the pins and ground.

Check the resistance of the MAF signal wiring. Pin D on the MAF and pin 50 on the computer
(dark blue/orange wire) should be less than 2 ohms. Pin C on the MAF and pin 9 on the
computer (tan/light blue wire) should be less than 2 ohms.

There should be a minimum of 10K ohms between either pin C or D on the MAF wiring
connector and ground. Make your measurement with the MAF disconnected from the wiring harness.

The computer sees a lean mixture signal coming from the O2 sensors and tries to
compensate by adding more fuel. Many times the end result is an engine that runs pig
rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the
ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage
is generated between the two sides of the tip. The voltage is greater than 600 millivolts.
Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low
as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get
smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never
measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter
with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will
average a changing voltage." End Quote

Testing the O2 sensors
Measuring the O2 sensor voltage at the computer will give you a good idea of how well
they are working. You'll have to pull the passenger side kick panel off to gain access to the
computer connector. Remove the plastic wiring cover to get to the back side of the wiring.
Use a safety pin or paper clip to probe the connections from the rear.
The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark
blue/pink wire). Use the ground next to the computer to ground the voltmeter.

Note that all resistance tests must be done with power off. Measuring resistance
with a circuit powered on will give false readings and possibly damage the meter.
Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are
disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for
the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

What to look for to fix your problem:
The 51 & 54 codes both point to a bad computer signal ground. This would allow the
voltage to be higher that it should be normally.The black/white wire is signal ground for
the TPS, EGR, ACT, ECT and BARO signals. The BARO sensor hasn't poped up a
code, so the problem is in the engine wiring harness, most likely in the 10 pin connectors.

Clean the 10 pin connectors with electronic parts cleaner or non-inflammable brake
parts cleaner (same stuff in a bigger can and cheaper too). The white connector center pin
marked SIG-RTN in the diagram is the black/white wire signal ground. Use a test light
with one lead connected to the battery and the other to the black/white wire on
the TPS or ECT. You will probalbly need to disconnect the sensor to get a good connection.
Wiggle the wiring and the 10 pin connectors and watch to see if the light flickers. If it still
flickers after cleaning the 10 pin connectors, you have a broken black/white signal ground wire.

MAF code 66: Do you have a cold air intake system other than the factory stock system?
These can cause problems and the fix is often to rotate the MAF housing to smooth
out the airflow through the MAF.

If you measured the MAF output voltage across the C & D pins like it is supposed to be
done and got .86 volts, you may have wiring problems. Never connect the voltmeter
leads to ground when checking the MAF. Disconnect the MAF connector and then
access the computer by removing the passenger side kick panel and disconnect the computer connector.
There is a 10 MM bolt that holds it in place. Measure the resistance between pin 9
on the compute and pin C on the MAF wiring harness connector. Measure the resistance
between pin 50 on the compute and pin D on the MAF wiring harness connector. Both should
read less than 1.5 ohms. More than than that is an indication of wiring problems.

The following is a view from the computer side of the computer connector.

The code 91 is probably a vacuum leak. However, it may disappear after you fix the MAF problems.

SN Certified TechnicianFounding Member

I have basically quit answering requests for help via PM's and emails. I find that the input and insight
that other people bring to the discussion is very valuable. They will often see or think of things that
did not occur to me.

The following description works on the basis of you having an understanding of basic electrical
circuits. If you don't understand it, then trying to explain what electricity is, what resistance is
and how electricity flows through a circuit is beyond what can be done in this reply.

The 51 & 54 codes point to a loss of signal ground. The TPS sensor is a variable resistor with one leg
tied to VREF and the other leg tied to ground. Its function is to provide a means of supplying a voltage
that increases as the throttle opens. With a missing or poor quality signal ground, there is no way for the
voltage to decrease to the level that tells the computer that the throttle is closed and it should act
accordingly. With a poor ground, instead of seeing .7 volts, it might see 1.5 volts and think you were
cruising and increase the injector pulse width and airflow through the IAB.

As to where you can find the signal ground wire, the black/white wire is signal ground for the TPS, EGR,
ACT, ECT and BARO signals. Look closely at the diagram Tmoss provided that shows the whole
computer system layout. You will see that the black/white signal ground is connected to a number of things.

In your case, the TPS, ACT & ECT all seem to have a loss of signal ground. That means the problem is
limited to the engine fuel injector harness. The 10 pin connector on the engine side of the fuel injector
harness is a good place to start a careful inspection for corrosion, broken wiring, and bad connections.
The wiggle test using a test lamp would be very useful in locating the problem.

Just finished cleaning the hell outta the 10 pin white connector and i did the test light idea you had jritcher, and it seems like i might have an intermittent ground problem. It looks like when i wiggle the wires a certian way the test light does flash. i took the tape and the wire loom off and inspected the wires and from what i can see all looks good. its very strange. i then put everything back 2gether and start the car up and played with the wires while it was running and you could hear a few times the car idle flucuated a bit and made some noises.

This part of the connector (wiring) does'nt seem to be the problem, it's seems like its the part that connects to the picture below:

Founding Member

waow i pretty much have the EXACT same problem, just finished replacing my ACT ECT & 02's and a new IAC. Still idles higher than it should and stutters when engine is cold. Im almost certain its a vacuum leak somwhere. Im just waiting on someone to have a vac gauge for me to hook up.

SN Certified TechnicianFounding Member

It is possible that the black/white wire has broken inside the insulation. Often wires will break inside the insulation
and will only become evident when you pull on them and the wire stretches. You see the wire get longer as you pull
on it, and you know that the copper does not stretch like plastic. In that case, you end up replacing the wire.

Sometimes the crimp on pin will go bad from oil getting inside between the crimp part of the pin shell and the copper
wire. Remove the wire from the plastic connector before doing anything else. Cleaning them with brake parts cleaner
and then trying to solder them is the fix. Have some heat shrink tubing handy in case you melt some of the wire insulation.

waow i pretty much have the EXACT same problem, just finished replacing my ACT ECT & 02's and a new IAC. Still idles higher than it should and stutters when engine is cold. Im almost certain its a vacuum leak somwhere. Im just waiting on someone to have a vac gauge for me to hook up.

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Good luck. im pretty positive its my SIG-RTN wire i think so at least.

SN Certified TechnicianFounding Member

I will have to spend some time looking at the connector and pin arrangement. On many ofthe connectors, you pry out an insert and then you can remove the pins. I haven't had a 10 pin taken apart in over18 months, so my memory isn't up to the give you good directions point.

BigHairyMonkey said:

waow i pretty much have the EXACT same problem, just finished replacing my ACT ECT & 02's and a new IAC. Still idles higher than it should and stutters when engine is cold. Im almost certain its a vacuum leak somwhere. Im just waiting on someone to have a vac gauge for me to hook up.

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Don't jump on the same wagon as someone else just because it sounds the same. There was a lot of digging and testing to get to this point. Start your own post and dump the codes to see what you get.